Suppose you shot DVCAM onto a tape, then shot miniDV over it. At the transition points, you will get a very high number of dropouts (very, very obvious). There'll also be a TC break at the end. At other points in the footage, some of the original DVCAM material may "leak" through.

Never heard of this "leak through" as the signal strength on the tape is the same in DV and DVCam. The breakup you are going to see is as the deck is switching mdes internally from higher to slower or slower to higher data rates.

You will also have an imediate tape speed change which may cause other issues as DVCam is 1/3 faster than MiniDV. In that changeover point, you are asking your deck to either speed up or slow down as well as change video stream speeds.

Not a good idea to record differing formats on one tape...I have done it too. It shouldn't hurt anything. Once it locks speed and formats, you are good to go again.